Zeolite adsorbents, method for obtaining them and their use for removing carbonates from a gas stream

ABSTRACT

The present invention relates to a novel family of zeolite adsorbents comprising a mixture of zeolite X and zeolite LSX, these adsorbents being predominantly exchanged with sodium or with strontium. These adsorbents are particularly suited to the decarbonatation of gas flows contaminated by CO 2 .

DESCRIPTION

1. Field of the Invention

The field of the invention is that of zeolite adsorbents for thepurification of gas flows contaminated by carbon dioxide, in particularfor the purification of air before N₂/O₂ separation stages.

2. Background of the Invention

The production of pure gases, in particular N₂ and O₂, from atmosphericair is an industrial operation carried out on a large scale and can makeuse either of cryogenic processes or of adsorption processes based onthe principle of pressure swing adsorption (PSA), that of temperatureswing adsorption (TSA) or a combination of the two (PTSA). Furthermore,many gases resulting from industrial processes comprise significantamounts of carbon dioxide, which is often advisable to remove.

The production of N₂ or O₂ from air requires a purification prior to theseparation stage proper. This is because, in carrying out cryogenicprocesses, water or carbon dioxide present in the feed air can result Inblockages of the equipment due to the fact that these operations arecarried out at temperatures far below the freezing points of theseimpurities. In the adsorption processes, water and carbon dioxide aremore strongly adsorbed than nitrogen and result, in the long run, inpoisoning of the adsorbent, the consequence of which is a decrease inthe expected life time.

In these processes, a zeolite of faujasite type (13X, the Si/Al ratio ofwhich is greater than 1.2) is very generally employed to Provide for theremoval of the carbon dioxide, the trapping of the water generally beingcarried out on an alumina bed place upstream of the bed of zeoliteadsorbent. The regeneration of the adsorbent is of PTSA type, that is tosay that a slight rise in temperature to approximately 150° C. isCombined with a reduction in pressure. During the stage, a fraction ofpurified gas produced which comprises N₂, O₂ and approximately 1% byvolume of argon, is conveyed to the beds of adsorbents for the purposeof regenerating them by desorbing CO₂ and H₂O.

It is has been known for a long time that zeolite X is a betteradsorbent for carbon dioxide than silica gel or active charcoal (U.S.Pat. No. 2,882,244). This patent also teaches that the selectivity withrespect to various adsorbents varies with the temperature and thepressure.

U.S. Pat. No. 3,885,927 teaches that the adsorption of CO₂ can becarried out on a zeolite X exchanged to more than 90% with barium: underthese conditions, the CO₂ content of the gas to be purified does notexceed 1000 ppm and the temperature can be between −40° C. and 50° C.

EP 294 588 teaches that a zeolite X exchanged with strontium, preferablyto 70%, can also be used to carry out this purification.

The influence on CO₂ adsorption of the number of exchangeable cations onthe zeolite has been studied by Barrer et al. in “Molecular Sieves”(Soc. Chem. Ind., London, 1968), p. 233, and by Coughlan et al. in “J.C. S. Faraday”, 1, 1975, 71, 1809. These studies show that theadsorption capacity of the zeolite for CO₂ increases as the Si/Al ratiodecreases, up to a limit of 1.2, the lower range not having beenexplored.

Zeollte X, the Si/Al ratio of which is close to 1.25 and which iscommonly used, is very selective for CO₂, this selectivity increasing asthe temperature falls. At temperatures in the region of ambienttemperatures, the efficiency decreases greatly as a result of thecompetition with nitrogen, which is present in much greater molarproportions. The N₂/CO₂ ratio in ambient air (with CO₂˜300/400 vpm) isof the order of 3000.

U.S. Pat. No. 5,531,808 discloses the teaching that CO₂ can be veryefficiently adsorbed by means of a zeolite of X type having an Si/Alratio of less than 1.15 and preferably equal to or very close to 1,referred to in the continuation of the account as zeolite LSX (LowSilica X). The advantage with respect to the conventional zeolite X(Si/Al>1.2) lies in the fact that it is no longer necessary to decreasethe temperature at the decarbonatation stage by means of a cold unit asthe efficiency of the zeolite is such that the selectivity for CO₂ withrespect to nitrogen remains high, even up to 50° C.

The Applicant Company has found that the CO₂ adsorption capacity of azeolite NaLSX increases with the degree of exchange with sodium but alsothat the increase in efficiency begins to reach a ceiling when degreesof exchange with sodium are achieved which are of the order of 90% forrelatively high CO₂ partial pressures. On the other hand, the ApplicantCompany has shown, in WO 99/46031, that a very substantial increase inefficiency can be obtained for The decarbonatation under low CO₂ partialpressures, of the order of 2 mbar, with zeolites LSX having a degree ofexchange with sodium (defined as the molar ratio of the sodium ions tothe aluminium atoms in the tetrahedral position, the remainder beingpotassium) of at least 98%.

DESCRIPTION OF THE INVENTION

A subject-matter of the present invention is a novel family of zeoliteadsorbents comprising a mixture of 5% to 95% and preferably of 50 to 90%by weight of at least one zeolite X with an Si/Al ratio equal to 1.25and of 95 to 5% and preferably of 50 to 10% by weight of at least onezeolite LSX with Si/Al=1 for which

either at least 80% of the sum of the exchangeable cationic sites of allof the zeolites of the mixture are occupied by sodium cations,

or at least 70% of the sum of the exchangeable cationic sites of all ofthe zeolites of the mixture are occupied by strontium cations, it beingpossible for the remainder of the exchangeable sites to be occupied bycations chosen from Groups IA, IIA and IIIA of the Periodic Table ortrivalent ions from the rare earth or lanthanide series.

Mention will very particularly be made, among preferred adsorbents, ofthose with an overall degree of exchange with sodium of greater than 90%and advantageously greater than 98%. Mention will also be made ofmixtures of zeolite adsorbents as defined above exchanged to at least70% with strontium, the majority of the remaining cationic sites ofwhich are occupied by sodium ions.

These novel zeolite adsorbents can be provided in the form of a powderbut they can also be agglomerated in the form of beads or extrudateswith 5 to 25, preferably 5 to 20, parts by weight of an inertagglomeration binder (amorohous material with a cohesive nature whichhas very little tendency to adsorb carbon dioxide) per 100 parts byweight of mixture of zeolite X and zeolite TSX and of binder.

The agglomerates are particularly well suited to industrial uses insofaras their handling during loading and unloading operations in anindustrial unit limits the pressure drops with respect to adsorbents inthe pulverulent form.

Another subject-matter of the present invention is the process for thepreparation of the adsorbents as defined above.

When the adsorbents are provided in the pulverulent form, they can beobtained by simple mixing of zeolite X and zeolite LSX powders.

Synthetic zeolite X and zeolite LSX powders generally exhibit a degreeof exchange with sodium of 100% and 77% respectively, the remainder ofthe cationic sites being essentially potassium ions.

These powders can be subjected to one or more optional cationicexchanges, either separately (i.e. prior to the intimate mixing thereof)or subsequent to the mixing stage.

These cationic exchanges consist in bringing the powders into contactwith saline solutions of the cation or cations which it is desired topartially or completely insert in the zeolite structure or structures inplace of the exchangeable cations already present.

Degrees of exchange are generally obtained in the conventional manner bycarrying out successive exchanges with the saline solution or solutionsof cations.

When the powders comprise a mixture of cations, the exchange can becarried out either via a mixed solution comprising salts of severalcations or by successive exchanges of individual saline solutions, inorder to insert the cations one after the other.

When the adsorbents are provided in the form of agglomerates, the stagesin the production process are generally as follows:

A—Agglomerating and shaping the mixture of X and LSX powders with abinder,

B—Drying at low temperature (of the order of 80-100° C.) and activatingat a temperature of between 300 and 700° C., preferably between 400 and600° C., the product obtained in A),

C—Optional zeolitization of the binder, if the binder can be convertedto a zeolite,

D—Washing, drying and activating, at a temperature of between 300 and700° C., preferably between 400 and 600° C., the product obtained in C)or the product obtained after cationic exchange of the product resultingfrom B).

Mention may be made, as examples of an inert binder, of silica, aluminaor clays and, as binder which can be converted to a zeolite, kaolin,metakaolin or halloysite.

The constituents of these agglomerates can be subjected to one or morecationic exchanges, followed by washing with water,

either before stage A), as indicated above for the pulverulent mixtures;in this case, the agglomerates are obtained on conclusion of stage B) orD), depending upon whether there is or is not zeolitization of thebinder,

or after stage B),

or after the optional stage of zeolitization of the binder which can beconverted to a zeolite on the predried products resulting from stage C)and before stage D).

If there s neither cationic exchange nor zeolitization, the adsorbentaccording to the invention is obtained on conclusion of stage B).

An alternative form of stage A) consists in conventionally mixingcrystalline zeolite X and zeolite LSX powders with water and a binder(generally in powder form) and in then spraying this mixture overalready formed zeolite agglomerates which act as agglomeration seeds.During this spraying, the agglomerates can be subjected to a continuousrotation about themselves according to a “snowball” type technique, forexample in a reactor equipped with a rotational axis. The agglomeratesthus obtained are then provided in the form of beads.

The zeolitization stage (stage C)) consists in converting the binderwhich can be converted to a zeolite, with which the mixture of zeoliteLSX and zeolite X powders has been agglomerated beforehand, by alkalinesteeping, for example according to the process disclosed in PatentApplication WO 99/05063, thus making it possible to obtain agglomeratescomprising little material which is inert with regard to adsorption,typically up to approximately 5% by weight of inert binder afterzeolitization, which exhibits an undeniable advantage during the use ofsuch adsorbents.

Another subject-matter of the invention is a process for thedecarbonatation of a gas flow. The decarbonatation process according tothe invention can be carried out by passing the gas flow to bedecarbonatated over one or more adsorbent beds combined in parallel orcapable of linking together the adsorption stage and the desorptionstage (intended for the regeneration of the adsorbent) in a cyclicalfashion; at the industrial stage, it is preferable to operate accordingto a process of adsorption by varying the pressure (PSA) andadvantageously of adsorption by varying the pressure and the temperature(PTSA). Processes of PSA and PTSA type involve the use of pressurecycles. In a first phase, the adsorbent bed provides for the separationof the contaminant by adsorption of this constituent; in a second phase,the adsorbent is regenerated by reducing the pressure. At each newcycle, it is essential for the desorption of the contaminant to be ascomplete and as efficient as possible, so as to recover a regeneratedstate of the adsorbent which is identical or substantially identical ateach new cycle.

The partial pressure of the CO₂ present in the gas flow generally doesnot exceed 25 mbar and is preferably less than 10 mbar.

So as to continuously purify the gas flow, such as air, a number ofadsorbent beds are generally positioned in parallel and are subjectedalternately to a cycle of adsorption with compression and of desorptionwith decompression. In the PSA and PTSA processes, the treatment cycleto which each bed is subjected comprises the following stages:

a) passing the contaminated gas flow into an adsorption regioncomprising the adsorbent bed, the adsorbent bed providing for theseparation of the contaminant or contaminants (in this instance CO₂) byadsorption,

b) desorbing the adsorbed CO₂ by establishing a pressure gradient andgradually reducing the pressure in the adsorption region in order torecover the CO₂, via the inlet into the adsorption region

c) increasing the pressure in the adsorption region by introducing apure gas stream via the outlet of the adsorption region.

Thus, each bed s subjected to a treatment cycle comprising a phase ofproducing pure gas, a second phase of decompression and a third phase ofrecompression.

If the only contaminant to be removed from the gas flow is CO₂, only oneadsorbent bed, composed essentially of agglomerates as defined above, isplaced in the adsorption region.

If there are several contaminants to be removed, the adsorption regioncan then comprise several adsorbent beds capable of adsorbing theundesired impurities or contaminants. Thus, in order to remove thecarbon dioxide and water present in air, a drying agent for adsorbingwater, such as alumina or a silica gel, will be combined with thezeolite adsorbent of the present invention.

So as to optimize the PSA and PTSA processes, the phases ofdecompression and of compression of the various adsorbent beds aresynchronized: it proves to be particularly advantageous to introducestages for the equalization of the pressures between two adsorbent beds,one being in the decompression phase and the other in the recompressionphase.

During the implementation of the process according to the invention, theadsorption pressures are generally between 0.2 and 20 bar and preferablybetween 1 and 10 bar, whereas the desorption pressures are generallybetween 0.02 and 5 bar and preferably between 0.1 and 2 bar.

As for the decarbonatation processes of the state of the art, thetemperatures n the adsorption region are generally between 20 and 80° C.and advantageously between 30 and 60° C.; in the decarbonatationprocesses of the stare of the art, the regeneration temperatures whichare necessary in order to obtain sufficient regeneration of theadsorbent are typically of the order of 130 to 170° C., which makes itnecessary to heat the adsorbent and increases the cost of the industrialplant.

With respect to the state of the art, the present invention offers asubstantial additional advantage as regards the regeneration of thezeolite adsorbents agglomerated with a zeolitized binder according tothe invention, insofar as, in order to obtain the same performance fromthe adsorbent after it has been regenerated, the regenerationtemperatures to be employed are between 100 and 120° C. and are thusmuch lower than those used to date.

EXAMPLES Example 1

Preparation of an adsorbent by mixing LSX and 13 X powders, followed byagglomeration and Na exchange.

The first stage consists in preparing the mixture composed

of 65% by weight of anhydrous zeolite X powder (Si/Al=1.25; degree ofexchange with sodium in the region of 100%), the adsorption capacity ofwhich for toluene at a relative pressure of 0.5 and at 25° C. is between23.5 and 24.5%,

and of 35% by weight, of anhydrous zeolite LSX powder (ratio Si/Al=1;degree of exchange with sodium 77%), the adsorption capacity of whichfor toluene at a relative pressure of 0.5 and at 25° C. is between 22and 23% .

This mixture is subsequently agglomerated and shaped into beads byaddition of 15 parts by weight of a clay per 85 parts by weight of themixture of zeolites. The agglomerates are subsequently dried at atemperature of the order of 80-100° C. and activated at 500-600° C. Thelatter are subsequently brought into contact several times with a 2Msodium chloride solution at 80° C. for 4 h in order to increase thedegree of exchange with sodium. At each stage, the ratio of volume ofsolution to mass of solid is 7 ml/g. Between each exchange, the solid iswashed several times, so as to remove excess salts therefrom.

After a single exchange, the overall degree of exchange with sodium(measured by X-ray fluorescence or by conventional chemical attackaccording to a plasma ionization technique (ICP, inductibly coupledplasma)) is equal to 94% and after 4 exchanges it reaches 99%. Theagglomerates, thus exchanged, are subsequently dried at low temperatureand activated at 500-600° C. The overall Si/Al ratio of the zeolitematerial of these adsorbents is equal to 1.17.

Their adsorption capacity for CO₂, expressed in cm³/g at 25° C. undervarious CO₂ pressures, is measured, as is their adsorption capacity fortoluene at 25° C. under a partial pressure of 0.5, which is 20-21%.

By way of comparison, the adsorption capacity of beads of zeolite Xagglomerated with 15% of the same binder is measured, as is theadsorption capacity of beads of zeolite NaLSX (degree of exchange withsodium 94%) agglomerated in an identical way. The results are combinedin Table 1.

The water content of the agglomerates, measured by coulometry, isbetween 0.1 and 0.3% of the total weight of the agglomerates.

Table 1 also shows the theoretical degree of exchange with sodium of amixture of 65% of zeolite NaX (degree of exchange with sodium in theregion of 100%) and of 35% of zeolite LSX (degree of exchange withsodium equal to 94%), as well as its theoretical adsorption capacity,which is calculated according to the partial pressure law.

TABLE 1 Degree of Pressure exchange (mbar) Type of beads with Na (%) 2 510 CO₂ adsorption NaX beads 100 14 25.1 34.5 NaLSX beads 94 29.3 49.365.2 Theoretical 98 19.3 33.6 45.2 calculation from lines 1 and 2 intable Beads according 94 24 35.4 45.4 to the invention (NaX + NaLSX)Beads according 99 31.5 40.2 48.9 to the invention (NaX + NaLSX)

It is found that the agglomerates according to the invention adsorb atvery low pressure (2-5 mbar) at least 24% more CO₂ in comparison withthe theoretical mixture of X and LSX beads.

Example 2

Preparation of an adsorbent by mixing LSX and 13X powders, followed byNa exchange and agglomeration.

The mixture of zeolite X and zeolite LSX powders of Example 1 is broughtinto contact several times with a 2M sodium chloride solution at 80° C.for 4 h in order to increase the degree of exchange with sodium. At eachstage, the ratio of volume of solution to mass of solid is 7 ml/g.Between each exchange, the powder is washed several times so as toremove excess salts therefrom.

After 4 exchanges, the overall degree of exchange with sodium of thepowder mixture is equal to 99%. This mixture is subsequentlyagglomerated in the form of beads by addition of 15 parts by weight ofclay per 85 parts by weight of zeolite powder mixture, then dried at80-100° C. and activated at 500-600° C. The overall degree of exchangewith sodium of these agglomerates is equal to 99%. The overall Si/Alratio of the zeolite material of these adsorbents is equal to 1.17 andtheir water consent is in the same range as that of the agglomerates ofExample 1.

The adsorption capacity for CO₂, measured under the same conditions asdescribed in Example 1, is identical to that of the adsorbent of Example1, he overall degree of exchange with sodium of the zeolite material ofwhich is equal to 99%.

Example 3

Preparation of an adsorbent by Na exchange on an LSX powder, then mixingwith a 13X powder with a degree of exchange with Na in the region of100%, then agglomeration.

The first stage consists in bringing zeolite LSX powder (Si/Al=1; degreeof exchange with sodium 77%) into contact with a 2M sodium chloridesolution at 80° C. for 4 h in order to increase the degree of exchangewith sodium. At each stage, the ratio of volume of solution to mass ofsolid is 7 ml/g. Between each exchange, the solid is washed severaltimes so as to remove excess salts therefrom.

After 4 exchanges, the overall degree of exchange with sodium of thepowder mixture is equal to 99% .

The second stage consists in mixing 35% by weight of zeolite NaLSXpowder exchanged in stage 1 with 65% of zeolite X powder (Si/Al=1.25;degree of exchange with sodium in the region of 100%) and in thenagglomerating, in the form of beads, 85 parts by weight of thispulverulent mixture with 15 parts by weight of a clay. The agglomeratesare subsequently dried at 80-100° C. and calcined at 500-600° C. Theirwater content is in the same range as that of the agglomerates ofExample 1.

The adsorption capacity for CO₂ of the adsorbent, measured under thesame conditions as those described in Example 1, is identical to that ofthe adsorbent of Example 1, the overall degree of exchange with sodiumof which is also in the region of 99%.

Example 4

Preparation of an adsorbent by mixing agglomerated beads of zeolite Xand zeolite LSX, followed by Na exchange on the mixture of beads.

The adsorbent is in this instance obtained by mixing

65% by weight of beads of zeolite X (Si/Al=1.25; degree of exchange withNa in the region of 100%) which are agglomerated with 15 parts by weightof clay per 85 parts by weight of zeolite X and 35% by weight of beadsof zeolite LSX (Si/Al=1; degree of exchange with Na in the region of77%) which are agglomerated with 15 parts by weight of clay per 85 partsby weight of zeolite LSX.

The mixture of beads is dried at 80-100° C. and then activated at500-600° C. before being brought Into contact with a 2M sodium chloridesolution at 80° C. for 4 h in order to increase the degree of exchangewith sodium. At each stage, the ratio of volume of solution to mass ofsolid is 7 ml/g. Between each exchange, the beads are washed severaltimes so as to remove excess salts therefrom. After 4 exchanges, theoverall degree of exchange with sodium of the beads is equal to 99%. Thebeads are subsequently dried at 80-100° C. and then activated at500-600° C. Their water content is in the same range as that of thebeads of Example 1. The adsorption capacity for CO₂ (expressed in cm³/gat 25° C.) is measured under various CO₂ pressures; the results arecombined in Table 2.

TABLE 2 99% degree of exchange Pressure with Na 2 mbar 5 mbar 10 mbarCO₂ adsorption 21.6 33 41

Example 5

Preparation of an adsorbent by Na exchange on agglomerated beads ofzeolite LSX, followed by mixing with agglomerated beads of zeolite X.

The adsorbent is obtained by mixing, by weight

65% of beads of zeolite X (Si/Al=1.25) agglomerated with 15 parts byweight of clay per 85 parts by weight of zeolite X

and 35% of beads of NaLSX (Si/Al=1; degree of exchange with Na In theregion of 99%).

The mixture is dried at 80-100° C. and then activated at 500-600° C.Their water content is in the same range as that of the beads of Example1.

The adsorption capacity for CO₂ of these beads is identical to that ofthe beads of Example 4 (cf Table 2).

Example 6

Preparation of an adsorbent by mixing LSX and 13X powders, followed byagglomeration with a binder which can be converted to a zeolite,zeolitization of the binder and Na exchange.

85 parts by weight of the mixture of powders resulting from stage 1 ofExample 1 are agglomerated here with 15 parts by weight of kaolin claywhich can be converted to a zeolite in the form of beads. Theagglomerates are subsequently dried at 80-100° C., then calcined at500-600° C., then immersed in an aqueous sodium hydroxide solution witha concentration of 220 g/l for 3 h and then washed with water accordingto the procedure disclosed in WO 99/05063.

Their adsorption capacity for toluene, measured at 25° C. under apartial pressure of 0.5, is 22.5-23%, which corresponds to a level ofbinder which does not exceed 5% of the total weight of the agglomerates.The beads are subsequently dried and brought into contact with a 2Msodium chloride solution at 80° C. for 4 h in order to increase thedegree of exchange with sodium. At each stage, the ratio of volume ofsolution to mass of solid is 7 ml/g. Between each exchange, the beadsare washed several times so as to remove excess salts therefrom. After 4exchanges, the overall degree of exchange with sodium of the beads isequal to 99%. The beads are subsequently dried at 80-100° C. and thenactivated at 500-600° C. Their water content is in the same range asthat of the beads of Example 1.

The results obtained for CO₂ adsorption capacity with regard to thesebeads, expressed in cm³/g at 25° C., under various CO₂ pressures arepresented in Table 3.

TABLE 3 99% degree of exchange Pressure (mbar) with Na 2 5 10 CO₂adsorption 32.3 43.8 53.8

Example 7

Preparation of an adsorbent by mixing beads of zeolite LSX agglomeratedwith zeolitized binder and beads of zeolite X agglomerated withzeolitized binder, followed by Na exchange on the mixture of beads.

The first stage consists in mixing 65 parts by weight of anhydrous beadsof zeolite X (Si/Al=1.25; degree of exchange in the region of 100%) and35 parts by weight of anhydrous beads of zeolite LSX (Si/Al=1; degree ofexchange with Na in the region of 77%). These beads of zeolite X andzeolite LSX, each comprising 5% of binder, were obtained according tothe process disclosed in Application WO 99/05063. The mixture of beadscomprising 5% of binder is dried at 80-100° C. and then calcined at500-600° C. The mixture is brought into contact with a 2M sodiumchloride solution at 80° C. for 4 h in order to increase the degree ofexchange with sodium of the zeolite material. At each stage, the ratioof volume of solution to mass of solid is 7 ml/g. Between each exchange,the beads are washed several times so as to remove excess solidstherefrom. After 4 exchanges, the overall degree of exchange with sodiumof the beads is equal to 99%. The beads are subsequently activated at500-600° C.

The adsorption capacity for CO₂ and the water content of these beads areidentical to those of the beads of Example 6 (cf. Table 3).

Example 8

Preparation of an adsorbent by mixing beads of zeolite LSX agglomeratedwith a binder which can be converted with a zeolite and beads of zeoliteX agglomerated with a binder which can be converted to a zeolite,followed by zeolitization of the binder and Na exchange on the mixtureof beads (alternative form of Example 7).

The adsorbent is prepared by mixing, in a first step,

35% by weight of beads of zeolite LSX (Si/Al=1; decree of exchange withNa in the region of 77%) comprising 15 parts by weight of a kaolinbinder which can be converted to a zeolite per 85 parts by weight ofzeolite LSX

with 65% by weight of beads of zeolite X (Si/Al=1.25; degree of exchangein the region of 100%) comprising 15 parts by weight of a kaolin binderwhich can be converted to a zeolite per 85 parts by weight of zeolite X.

After drying at low temperature (80-100° C.) and then activating at500-600° C., the mixture is immersed in an aqueous sodium, hydroxidesolution (concentration 220 g/l) for 3 h according to the proceduredisclosed in Application WO 99/05363. The toluene adsorption capacity isthen measured on this mixture in order to evaluate the residual contentof binder therein, which is in the region of 5% of the total weight ofthe beads. The latter are then dried at 80-100° C., activated at500-600° C. and then brought into contact with a 2M sodium chloridesolution at 80° C. for 4 h in order to increase the degree of exchangewith sodium of the zeolite material. At each stage, the ratio of volumeof solution to mass of solid is 7 ml/g. Between each exchange, the beadsare washed several times so as to remove excess salts therefrom. After 4exchanges, the overall degree of exchange with sodium of the beads isequal to 99%. The beads are subsequently dried at 80-100° C. andactivated at 500-600° C.

The adsorption capacity for CO₂ and the water content of these beads areidentical to those of the beads of Example 6 (cf. Table 3).

Example 9

Preparation of an adsorbent by mixing beads of zeolite LSX agglomeratedwith a zeolitized binder, followed by Na exchange, and beads of zeoliteX agglomerated with a zeolitized binder.

In a first stage, beads of NaLSX comprising 5% of binder are prepared byagglomerating 85 parts by weight of a zeolite LSX powder (Si/Al=1;degree of exchange with Na in the region of 77%) with 15 parts by weightof a mixture composed of a clay of montmorillonite type (15% by weight),of a clay of kaolin type (85%), of a small amount ofcarboxymethylcellulose and of water. The agglomerates are dried at80-100° C. and calcined at 500° C. for two hours under an inertatmosphere which is devoid of water. These agglomerates are subsequentlyimmersed in a sodium hydroxide solution according to the teaching ofApplication WO 99/05063. They are then rinsed several times in water.Measurements of toluene capacity show that the residual level of binderis in the region of 5%.

The agglomerates are subsequently brought into contact with a 2M sodiumchloride solution at 80° C. for 4 h in order to increase the degree ofexchange with sodium of the zeolite material. At each stage, the ratioof volume of solution to mass of solid is 7 ml/g. Between each exchange,the beads are washed several times so as to remove excess saltstherefrom. After 4 exchanges, the overall degree of exchange with sodiumof the beads is equal to 99%. The beads are subsequently activated at500-600° C.

In a second stage, 35% by weight of NaLSX beads obtained on conclusionof the first stage are mixed with 65% of beads cf zeolite X agglomeratedwith a binder which has been zeolitized according to the processdescribed in Example 6, so that the content of inert material is in theregion of 5% of the weight of the beads.

The overall degree of exchange with sodium of the zeolite material ofthese beads is greater than 99%. They are then dried at 80-100° C. andcalcined at 500-600° C. The CO₂ adsorption capacities with regard tothese beads, measured at 25% under various CO₂ pressures, and theirwater content are identical to those of Example 6 (cf. Table 3).

Although the invention has been described in conjunction with specificembodiments, it is evident that many alternatives and variations will beapparent to those skilled in the art in light of the foregoingdescription. Accordingly, the invention is intended to embrace all ofthe alternatives and alterations that fall within the spirit and scopeof the appended claims. The foregoing references are hereby incorporatedby reference.

What is claimed is:
 1. Process for purifying air contaminated by CO₂ and H₂O, wherein gas flow to be purified is brought into contact, in an adsorption region, with at least one drying agent, and at least with one adsorbent composed essentially of a mixture of zeolite X and zeolite LSX, the overall degree of exchange with sodium of which is equal to or greater than 98%, agglomerated with a binder, the residual amount of inert binder in the adsorbent being less than or equal to 25 parts by weight per 100 parts by weight of mixture of zeolites and of binder, and with a water content advantageously representing at most 1% of the total weight of the adsorbent.
 2. Process according to claim 1, wherein the use of a treatment cycle comprises: a) passing the contaminated gas flow into an adsorption region comprising a bed of drying agent and a bed of adsorbent as defined in claim 1, b) desorbing the adsorbed CO₂ by establishing a pressure gradient and gradually reducing the pressure in said adsorption region to recover the CO₂ via the inlet into the adsorption region, c) increasing the pressure in said adsorption region by introducing a pure gas stream via the outlet of the adsorption region.
 3. Process according to claim 1, wherein the drying agent is based upon alumina and the amount of binder is
 20. 4. Process according to claim 1, wherein the amount of binder is
 5. 5. Process according to claim 1, wherein the amount of water is at most 0.5%.
 6. Zeolite adsorbent comprising a mixture of 5% to 95% by weight of at least one zeolite X with an Si/Al ratio equal to 1.25 and of 95 to 5% by weight of at least one zeolite LSX with Si/Al=1 wherein either at least 80% of the sum of the exchangeable cationic sites of all of the zeolites of the mixture are occupied by sodium cations, or at least 70% of the sum of the exchangeable cationic sites of all of the zeolites of the mixture are occupied by strontium cations, the remainder of the exchangeable sites to be occupied by cations selected from Groups IA, IIA and IIIA of the Periodic Table or trivalent ions from the rare earth or lanthanide series.
 7. Zeolite adsorbent according to claim 6, wherein the adsorbent is in the form of a powder of zeolite X and zeolite LSX.
 8. Process comprises producing an adsorbent in the form of a powder as defined in claim 7 by mixing zeolite X and zeolite LSX powders and by at least one optional cationic exchange, either on X and/or LSX powder prior to mixing thereof or subsequent to mixing thereof.
 9. Zeolite adsorbent according to claim 6, wherein the adsorbent is agglomerated with an inert binder and the amount of inert binder in the zeolite adsorbent is less than or equal to 25 parts by weight, per 100 parts by weight of the mixture of zeolites and of binder.
 10. Process for producing an agglomerated adsorbent as defined in claim 9, comprising: A)—agglomerating and shaping the mixture of X and LSX powders with a binder, B)—drying at low temperature and activating at a temperature of between 300 and 700° C., the product obtained in A), C)—optional zeolitization of the binder, if the binder can be converted to a zeolite, D)—washing, drying and activating, at a temperature of between 300 and 700° C., the product obtained in C) or the product obtained after cationic exchange of the product resulting from B), and optionally at least one cationic exchange, followed by washing with water: prior to stage A), either on the X and LSX powders prior to the mixing thereof or immediately after the mixing thereof wherein the agglomerated adsorbent is obtained on conclusion of stage B) or D) following the zeolitization or not of the binder, and/or after stage B), and/or after the optional stage of zeolitization of the binder which can be converted to a zeolite on the predried products resulting from stage C) and before stage D), if there is neither cationic exchange nor zeolitization, the agglomerated adsorbent is obtained on conclusion of stage B).
 11. Process according to claim 10, wherein activation is between 400 and 600° C. and washing between 400 and 600° C.
 12. Zeolite adsorbent according to claim 9, wherein the amount of binder is less than or equal to 20 parts.
 13. Zeolite adsorbent according to claim 9, wherein the amount is at the very most equal to 5 parts.
 14. Zeolite adsorbent according to claim 6, wherein the water content represents at most 1% of the total weight of the adsorbent.
 15. Zeolite adsorbent according to claim 14, wherein the amount of water is at most 0.5%.
 16. Zeolite adsorbent according to claim 14, wherein the amount of water is at most 0.3%.
 17. Process for the decarbonatation of a gas flow, contaminated by CO₂, comprising gas flow to be purified is brought into contact, in an adsorption region, with at least one zeolite adsorbent as defined in claim
 6. 18. Process for the decarbonatation of a gas flow according to claim 17 with a zeolite adsorbent, wherein the overall degree of exchange with sodium of which is greater than 90%.
 19. Process according to claim 18, wherein the degree of exchange is greater than 98%.
 20. Process for the decarbonatation of a gas flow according to claim 17 with a zeolite adsorbent, wherein the overall degree of exchange with strontium of which is greater than 70% and the majority of the remaining cationic sites of which are occupied by sodium ions.
 21. Process for the decarbonatation of a gas flow according to claim 17, wherein the operation is carried out by pressure swing adsorption (PSA) or by pressure and temperature swing adsorption (PTSA).
 22. Process according to claim 21, wherein adsorption is carried out at pressures of between 1 and 10 bar and desorption is carried out at pressures of between 0.1 and 2 bar.
 23. Process according to claim 17, further comprising the use of a treatment cycle comprising: a) passing the contaminated gas flow into an adsorption region comprising the adsorbent bed, the adsorbent bed providing for the separation of the contaminant or contaminants by adsorption, b) desorbing the adsorbed CO₂ by establishing a pressure gradient and gradually reducing the pressure in the adsorption region to recover the CO₂ via the inlet into the adsorption region, c) increasing the pressure in the adsorption region by introducing a pure gas stream via the outlet of the adsorption region.
 24. Process according to claim 23, wherein the zeolite adsorbent is agglomerated with a zeolitized binder, in which the adsorbent is regenerated, stage b), at a temperature of between 100 and 120° C.
 25. Process according to claim 17, wherein the gas is air and the adsorbent is agglomerated with a binder.
 26. Zeolite adsorbent according to claim 6, wherein the mixture comprises 50 to 90% by weight of the at least one zeolite X and 50 to 10% by weight of the at least one zeolite LSX. 